Nylons are polyamides which are generally synthesized by the condensation polymerisation of a diamine with a dicarboxylic acid, ω-aminoacids, or lactams. A common variant of nylon is nylon 6,6 which is formed by the reaction of hexamethylenediamine and hexane-1,6-dioic acid (adipic acid). Alternatively, nylon 6 may be synthesized by a ring opening polymerisation of caprolactam. Hence, hexane-1,6-dioic acid (adipic acid), aminohexanoic acid, hexamethyldiamine and caprolactam are all important intermediates in the production of nylons (Anton, A. and Baird, B. R. 2005. Polyamides, Fibers. Kirk-Othmer Encyclopedia of Chemical Technology).
Industrially, these intermediates have been synthesized using petrochemical-based feedstocks. In currently used commercial processes, the starting material for producing adipic acid and caprolactam is cyclohexane, which is then air oxidized in a series of steps to form a mixture of cyclohexanone (K) and cyclohexanol (A) wherein this mixture is known as KA oil. For adipic acid, the KA oil is oxidized in a subsequent step to adipic acid via nitric acid oxidation. For caprolactam, the cyclohexanone is converted to caprolactam through several well known steps (Oppenheim, J. P. and Dickerson, G. L. 2003. Adipic Acid. Kirk-Othmer Encyclopedia of Chemical Technology). The process and various improvements have been disclosed in U.S. Pat. Nos. 2,439,513; 2,557,282; 2,791,566; 2,840,607; 2,971,010 and 3,338,959.
In the air oxidation of cyclohexane to KA oil, significant volumes of waste streams are produced comprising of mixtures of dicarboxylic acids, monoacids, hydroxy acids, cyclohexanone, cyclohexanol, and esters/oligomers of such. These waste streams are called non-volatile residue (NVR), water wash, or COP acid streams. There have been numerous publications on chemical methods and processes for recovering material from these mixtures such as U.S. Pat. No. 4,058,555. However, the recovery of carbon from these streams suffers from low yields, and a large portion of the carbon is not utilized.
Economic and environmental considerations warrant more efficient carbon utilisation of the petrochemical feedstock cyclohexane. The utilization of the components in NVR from the caustic cyclohexyl hydroperoxide (CHHP) decomposition process as a fermentation substrate was investigated briefly decades ago (Ramsay et al., Applied & Environmental Microbiology, 1986, 52(1):152-156), but to date no biological processes for the treatment of these waste streams or for the utilization of the carbon in these waste streams had been developed due to the highly toxic nature of these streams to microorganisms. Thus, these waste streams need to be disposed of by burning and this may require combustion in specially designed incinerators dependent on the nature of the steams.
Accordingly, there is a need for alternative methods for utilizing the carbon lost in these waste streams more efficiently and/or converting the carbon into useful products, rather than burning it for fuel value or disposing of it.